Although mitochondrial abnormalities have been found in the muscle of some zidovudine-treated HIV+ patients with clinical myopathy (New England J. of Med. , 1990, 322: 1098), it is not clear if the myopathy results from zidovudine, the HIV infection, or both. To determine if zidovudine is myotoxic, the quadriceps muscle of Syrian hamsters given zidovudine 150 mg/kg body weight intraperitoneally twice daily was compared with control animals. Frozen sections showed an equivocal increase of formazan deposits after reduced nicotinamide adenine dinucleotide dehydrogenase-tetrazolium reductase histochemistry but no ragged-red fibers after modified trichrome stain. Electron microscopic examination revealed slight increases in the number and size of mitochondria of treated animals compared with controls. This finding was confirmed by the higher volume fraction of mitochondria in eight treated animals compared with six controls:9.2 plus or minus 1.5 volume percent versus 4.7 plus or minus 0.6 (p less than 0.05). SDS polyacrylamide gel electrophoresis of mitochondrial proteins showed several changes including a relative reduction of a 45-kd protein in the treated animals. These results show that zidovudine alone can cause mitochondriopathy in hamster skeletal muscle.